This invention relates to a method of producing a generally hemispherical bowl-like metal article such as, for example, a hollow hemispherical part of a pressure vessel or a rocket motor case. The metal material may be one that is inferior in formability, such as titanium or its alloy, and heat treatment of the bowl-like article is essential.
In many cases vessel-like metal structures such as pressure vessels and rocket motor cases have generally hemispherical or bowl-like parts. For example, a spherical pressure vessel or rocket motor case is usually produced by butt welding of two hemispherical parts along the equatorial edges, and in producing a cigar-like pressure vessel or rocket motor case a hemispherical part is welded at its equatorial edge to an open end of a cylindrical part. Recently such hemispherical parts are often made of titanium or its alloy which is superior in specific strength but is inferior in formability.
A typical conventional method of producing a spherical rocket motor case of a titanium alloy includes the following steps. A relatively thick plate of the titanium alloy is cut into a flat and circular blank, and the blank is drawn into a bowl-like shape by a hot press forming operation. In many cases some external parts are welded to the bowl-like work preceded by some preparatory machining operations. Next, the bowl-like work is subjected to a solution heat treatment and then to an age-hardening treatment. After that machining is made on the outer and inner surfaces of the bowl-like work until the wall thickness becomes as prescribed. The hemispherical part obtained through these steps is brought into abutment with another hemispherical part prepared in the same way, and the two parts are welded together to thereby obtain a spherical case. Sometimes the hemispherical parts are formed by forging instead of the aforementioned hot press forming.
The above described conventional method is not high in productivity and entails high material and labor costs. Because of inferior formability of the employed metal material it is impossible to accomplish the initial press forming operation with desirably high precision in the shape and dimensions of the hemispherically shaped work. Accordingly it is necessary to use a considerably thick blank compared with the wall thickness of the final product in order to allow redressing of irregularities by subsequent machining. Furthermore, oxide layers are formed on the surfaces of the hemispherical work which is exposed to the atmosphere during the heat treatment, and, besides, the work is liable to undergo some distortion during the heat treatment. For these reasons a considerable amount of the metal material must be removed from the heat-treated hemispherical work by machining on the outer and inner sides.